The present invention relates generally to optical transmission systems, and particularly to a method and apparatus for dynamically controlling chromatic dispersion and dispersion slope in optical communications systems using waveguide gratings.
Optical transmission systems, including optical fiber communication systems, have become an attractive alternative for carrying voice and data at high speeds. In optical transmission systems, waveform degradation due to chromatic dispersion in the optical transmission medium can be problematic, particularly as transmission speeds continue to increase.
Chromatic dispersion (CD) results from the fact that, in general, in transmission media such as glass optical fibers, the higher the frequency of the optical signal, the greater the refractive index. As such, higher frequency components of optical signals will xe2x80x9cslow down,xe2x80x9d and contrastingly, lower frequency signals will xe2x80x9cspeed-up.xe2x80x9d
In single mode optical fiber, chromatic dispersion results from the interplay of two underlying effects, material dispersion and waveguide dispersion. Material dispersion results from the non-linear dependence upon wavelength of the refractive index, and the corresponding group velocity, of the material, which is illustratively doped silica. Waveguide dispersion results from the wavelength dependent relationships of the group velocity to the core diameter and the difference in the index of refraction between the core and the cladding. Moreover, impurities in the waveguide material, mechanical stress and strain, and temperature can also impact the index of refraction, further contributing to the ill-effects of chromatic dispersion.
In digital optical communications, where the optical signal is ideally a square wave, bit-spreading due to chromatic dispersion can be particularly problematic. To this end, as the xe2x80x9cfast frequenciesxe2x80x9d slow down and the xe2x80x9cslow frequenciesxe2x80x9d in the signal speed up (or vice versa) as a result of chromatic dispersion, the shape of the waveform can be substantially impacted. The effects of this type of dispersion are a spreading of the original pulse in time, causing it to overflow in the time slot that has already been allotted to another bit. When the overflow becomes excessive, intersymbol interference (ISI) may result. ISI may result in an increase in the bit-error rate to unacceptable levels.
As can be appreciated, control of the total chromatic dispersion of transmission paths in an optical communication system is important, particularly in long-haul, and high-speed applications. To this end, it is necessary to reduce the total dispersion to a point where its contribution to the bit-error rate of the signal is acceptable. In commonly used dense wavelength division multiplexed (DWDM) optical communications systems, there may be 40 wavelength channels or more, having channel center wavelengths spaced approximately 0.8 nm to approximately 1.0 nm apart. Illustratively, a 40-channel system could have center wavelengths in the range of approximately 1530 nm to approximately 1570 mn. As can be appreciated, compensating for chromatic dispersion in such a system, and in a dynamic manner, can be difficult.
Another related phenomenon that can adversely impact signal transmission in an optical communications system especially at 40 Gb/s (or greater) transmission speeds is dispersion slope. Dispersion slope is the change (positive or negative) in dispersion versus wavelength. Sources of dispersion slope in optical communications systems include the dispersion from the optical waveguide (e.g. fiber); dispersion slope from optical components and equipment in the optical transmission system; and dispersion slope from thermal fluctuations. The control and correction of dispersion slope becomes increasingly important for high transmission rate systems operating with certain transmission formats. For instance, 40 Gb/s optical networks using RZ formats can be degraded when the received optical signal has 20 ps/nm2 or more of chromatic dispersion slope.
Interferometric devices have been employed to mitigate the ill-effects of chromatic dispersion. One such interferometric device that is currently attracting a great deal of attention in the optical communications industry is the chirped fiber Bragg grating (FBG). The principle of operation of the FBG is relatively straight-forward. A grating is written down the length of an optical fiber waveguide by periodically changing the refractive index of the waveguide core or cladding. Light in the fiber which has a wavelength that is two times the product of the grating period and the index of refraction is reflected.
By altering the period of the grating the length of the device (referred to as xe2x80x9cchirpingxe2x80x9d the grating), chromatic dispersion compensation can be realized. To this end, by chirping the grating, a relative wavelength-dependent delay may be introduced in the signal, enabling dispersion compensation. For example, if the period of the grating is reduced along the length of the device, blue light is reflected at a point farther into the device than red light, and is thus delayed relative to the red light. Alternatively, if the period of the grating were increased along the length of the device, the opposite relative delay would be had. As can be appreciated, the relative delay caused by chirped gratings can be used to compensate for chromatic dispersion present in an optical signal.
While the fiber Bragg gratings have been employed in dispersion compensation techniques, conventional Bragg grating-based dispersion compensators lack the capability of dynamically and tuneably controlling both dispersion compensation and dispersion slope in an optical signal.
What is needed, therefore, is a method and apparatus for controlling chromatic dispersion and dispersion slope in an optical signal that overcomes at least the drawbacks of conventional methods and apparati described above.
According to an exemplary embodiment of the present invention, an optical waveguide includes a grating which has a grating parameter that is adapted for dynamically variable non-uniform alteration. The non-uniform alteration of the grating parameter results in the introduction of a predetermined amount of chromatic dispersion and dispersion slope into an optical signal traversing the waveguide.
According to another exemplary embodiment of the present invention, a method of introducing chromatic dispersion and dispersion slope to an optical signal includes providing a waveguide having a grating parameter which is adapted for dynamically variable non-uniform alteration. The method further includes selectively varying the grating parameter to introduce a predetermined amount of chromatic dispersion and a predetermined amount of dispersion slope into an optical signal traversing the waveguide.
According to another exemplary embodiment of the present invention, an optical apparatus includes a plurality of optical waveguides each of which have an optical grating and respective grating parameters, and at least one of the waveguide gratings has a grating parameter that is adapted for dynamically variable non-uniform alteration. The optical apparatus further includes a device which dynamically varies the grating parameters of each of the waveguides to selectively introduce chromatic dispersion and dispersion slope into an optical signal traversing the apparatus.
According to another exemplary embodiment of the present invention, a method for introducing chromatic dispersion and dispersion slope in an optical signal includes providing a plurality of waveguides each having an optical grating with a respective grating parameters, and at least one of the waveguide gratings has a grating parameter that is adapted for dynamically variable non-uniform alteration. The method further includes dynamically varying the grating parameters of each of the waveguides to introduce a predetermined amount of chromatic dispersion, and a predetermined amount of dispersion slope into an optical signal traversing the waveguide.